PROJECT SUMMARY/ABSTRACT In the United States alone, Osteoporosis (OP) affects 5-8 million adults aged >50 years, with more than 44 million either having OP or being at high risk due to low bone mass. OP is associated with the majority of fractures in older adults and is a huge financial burden, costing the United States nearly $20 billion each year. Hip fractures are the most serious outcome of OP and 20% of patients will die within one year of fracture. Hip bone mineral density (BMD) is an important biomarker of fracture risk, but as a standalone measure it lacks information on modifiable causal factors. Convincing evidence indicates that physical activity is beneficial in prevention of osteoporotic fractures and that it is one of the main modifiable risk factors. As the number of older adults with OP in the United States is expected to rise to over 13 million by 2020, there is a critical need to develop an accurate portable low cost method for assessing modifiable factors of hip BMD in the free-living environment such as physical activity and dynamic skeletal loading to be used in combination with DXA scans. The major flaw with previous attempts to relate an index of cumulative loading to BMD is that they used methods with poor accuracy in measuring physical activity and dynamic loading. This project aims to further develop our innovative physical activity assessment algorithm, which is capable of accurately quantifying activity for gait velocities as low as 0.1 m/s using accelerometers, by combining it with a dynamic loading assessment algorithm to be developed in Aim 1 and applying it to relate free-living acceleration data to BMD (Aim 2). Therefore, the objective of this project is to provide a low cost portable tool to track physical activity and dynamic loading which can be used in combination with DXA scans to identify deficits in activity and mechanical loading, and to better inform subject-specific preventative treatment measures necessary to maintain or increase hip BMD and bone health. The rationale for this project is based on convincing evidence that daily physical activity with a higher frequency of peak hip accelerations ? 3.9g results in increases in hip BMD and that skeletal loading is highly correlated with acceleration.